This invention relates to an arrangement comprising a circuit for processing an AC signal which includes a first stage with a first active element biased with a first bias voltage applied between its first main electrode and its second main electrode, and with a second bias voltage applied between its control electrode and its second main electrode, the control electrode also receiving the AC signal and the active element being of the type in which, when in operation, normally no bias current flows in the control electrode.
The invention particularly finds its application in the realisation of semiconductor circuits operating in the very high frequency domain, e.g. in the field of telecommunications, in the range of consumer portable systems.
A power amplifier stage comprising field effect transistors is already known from the state of the art from U.S. Pat. No. 5,041,796. This stage is intended to form a broadband amplifier.
For this purpose, the active layer of the field effect transistor amplifier is modified relative to that of transistors habitually known to those skilled in the art. This modified layer presents a doping profile according to which the concentration of carriers is less strong near to the surface of the transistor and stronger as the distance to the surface becomes greater.
Said transistor is used with a bias arrangement which includes a resistor RG of 10 k.OMEGA. inserted between gate and ground, a resistor Rf of 300 .OMEGA. arranged in a series combination with a capacitor Cf of 50 pF inserted between its gate and drain, the load being constituted by an inductor placed between the drain and the positive DC power supply of 3 V, and the source of the transistor being connected directly to ground.
This transistor thus has the property of showing a constant transconductance as a function of the gate-source difference of potential; but it is not used for producing a DC potential of a different value from that of the DC power supply, which is +3 V, with a view to DC biasing various other stages of an integrated semiconductor circuit by means of this power amplifier stage.
Up till now a technical problem has arisen when one wishes to supply DC power to integrated semiconductor circuits of small portable systems, or of any consumer oriented and not very costly system. In these systems it is desirable to have only a single DC power supply for the system, for example, provided by 3 V batteries, and to generate in the circuit itself a second, negative, DC power supply which is necessary for the operation of further stages comprising the field effect transistors.
Up till now it has been known to those skilled in the art that an inverter stage may be used for generating a negative DC voltage from the +3 V DC supply voltage. This inverter stage carries out a sampling procedure while creating a high frequency AC signal having an amplitude between 0 and +3 V of a square wave shape. In response to this sampled signal a second shifting stage placed at the end of the inverter stage shifts the AC signal, which has an amplitude of 0 to 3 V by making the +3 V level correspond to ground, which brings the level which previously was 0 to a voltage of -3 V. In this manner a sampled signal between 0 and -3 V is produced which can generate a negative DC voltage of -3 V. The drawback is that this sampled signal has very steep edges and that it produces in fact all the multiples of the sampling frequency. This makes it necessary, in order to eliminate these highly annoying harmonics in the operation at high and very high frequencies, to add to this negative voltage generator circuit a filter whose structure is very sophisticated that is to say complex and costly.
Generally, it is a difficult problem to generate a second voltage having an opposite polarity to that of a first DC supply voltage or having a different value from that of this voltage in a high frequency or very high frequency integrated circuit, and a large number of circuits is involved in the solution to this problem.
In effect, in integrated circuits it is very often necessary to have two opposite-sign DC voltages to bias the transistors in each stage of the circuit. In the particular domain of very high frequency circuits to be applied to telecommunications, portable and consumer systems have appeared more and more, which systems are also required to be compact, that is to say, made with as small a number of elements as possible. They are to be highly reliable, not costly and, above all, simple, that is to say, operating with a single battery, thus having a single input for the DC power supply, but nevertheless operating with as good a performance if not better than the cumbersome sophisticated and costly circuits.